1. Field of Invention
The present invention is directed to multilayer injection molding apparatus, and more particularly, to multilayer injection molding apparatus having automatic process control.
2. Discussion of Related Art
Multilayer injection molding apparatus produce, for example, products that include at least one core (interior) layer and two skin (encapsulating) layers. The position of a core layer leading edge (i.e., the edges that are first injected) and the shape of the core layer in such a multilayer molded product is controlled, for example, by the amount of inner and outer skin layer material that is injected into the cavity before the core layer material begins to flow into the same cavity, and the volumetric flow rate of the materials.
One example of such a multilayer product is a blow molding preform. In such preforms, the position and shape of the core layer at least partially determines the performance of a subsequently formed blow-molded part. For example, the position and shape of the core may determine the gas permeability of the blow-molded part.
In multi-cavity, multilayer injection molding devices, the positions of the leading edges of the core layers of products produced in each of the cavities are preferably located within a predetermined range relative to the skin layer leading edge.
Conventionally known injection molding techniques suitable for controlling placement and quality of materials in multilayer products include thermally-balanced flow techniques and shooting pot techniques. In systems employing thermally-balanced techniques, the amount and timing of the introduction of the core materials and skin materials into the cavities are partially controlled by controlling the temperature of the skin material flow channels to a particular cavity relative to the skin material flow channels to the rest of the cavities, so that a suitable flow rate and volume of skin material flows into each cavity before injection of the core material begins. By contrast, in systems employing shooting pot techniques, shooting pots are used to determine the volume of core material and skin material fed into each cavity or group of cavities fed by that particular shooting pot. In injection molding apparatus employing shooting pots, the volumetric stroke in one or more skin shooting pots will alter the position of the leading edge in one or more cavities, and changing the volumetric stroke in all skin shooting pots will alter the position in all cavities.
Conventionally, the parameters in both thermally-balanced systems and shooting pot systems are set manually. That is, the molded products produced by each mold cavity are manually inspected (e.g., by cutting a cross section of a molded product) to determine the position of the leading edge. If the leading edge is not in the desired position, the machine operator adjusts parameters of the injection molding apparatus to bring the leading edge into a proper location. Setting the parameters is typically performed at the start of a molding run and whenever inspection of the molded products indicates that a molded product is outside of the predetermined acceptable range.
For example, in a thermally-balanced molding device producing a three-layer molded product, the parameters adjusted to achieve an acceptable leading edge position may include the temperatures of one or more of the nozzles that are used to inject the core material and the skin layer into the cavities of a mold, and may also include the start time of the injection of the core layer into the mold relative to the skin injection, such that the position of the leading edge of the core material in all cavities is affected.
An adjustment process typically takes several iterations, because an adjustment affecting a first cavity may have a secondary effect on surrounding cavities. The adjustments may take several hours for a skilled engineer and may take much longer for a less experienced operator. For the less skilled operators, a lack of understanding of the interaction of the process variables may even prevent them from successfully placing the leading edge in the desired position.
Once set, the leading edge position may be repeatably produced until some perturbation of the system occurs. Such perturbations may include, for example, changes in material properties due to the use of different lots of production of the core layer and/or skin layer materials, or changes in the moisture content of the materials, or changes in chilled water or tower water caused by diurnal changes in ambient air temperatures. Other perturbations may be caused by starting up or shutting down other machines in the plant that share the same utilities as the affected injection molding apparatus, or by perturbations in the auxiliary equipment used to dry the skin or core materials prior to molding, or by changes in temperature of a hydraulic fluid which may be used, for example, in the apparatus that injects mold material into the cavities of a mold. It is to be appreciated, that although monitoring of machine parameters and auxiliary equipment detects many perturbations, and allows them to be corrected by the machine operator, there are occasionally subtle perturbations that individually are small but which combine together to affect the leading edge position in a molded product, despite failing to exceed the “alarm limit” for any individual machine parameter.
Once an injection molding system is producing products having a leading edge in an acceptable location, the leading edge position is typically manually inspected on a continuous SPC (statistical process control) basis to determine if process adjustments are required. If the perturbations are minor, the leading edge may be stable within an acceptable range for days at a time, but if the perturbations are larger or the acceptable position range is selected to be narrow, the leading edge may require monitoring several times a day and appropriate machine adjustment may be performed as required.
Conventionally, injection molding machines suitable for producing blow molding performs have had forty-eight cavities. In an effort to increase the machine output per capital equipment cost to produce preforms, there is a desire to increase the number of cavities in molds, for example, to 144 cavities. As the number of cavities in multilayer molding systems has increased, the manual inspection, adjustment times and production rates have increased. Even with the use of robotically-fed ultrasonic or optical measurement systems, full inspection and measurement of a forty-eight cavity molding apparatus typically requires about one man-hour, and subsequent adjustment of the machine parameters or auxiliary equipment typically requires an additional hour. Of course, after such adjustments, SPC protocol requires more frequent inspection until a history of stability is reestablished.
With the introduction of higher cavitation systems such as 144-cavity systems, the manual inspection time is increased approximately threefold relative to forty-eight cavity systems. The amount of unqualified preforms produced during periods of adjustment is likewise increased as result of the increased adjustment time and the increased production rate. For example, in thermally balanced apparatus, during the first startup of a 144-cavity system, many tens of thousands of scrap parts may be manufactured at a significant cost.